I use it and I think it does work as I don't have a problem with flies in my bunny accommodation, just the odd one or two now and again in warm weather.
I do the same as Zoobec and put it on a cloth. I hang the cloth up on the mesh of the door, out of reach of the bunnies. I top it up with a few more drops every other day to keep the smell nice and strong.
You need to get the citronella essential oil, not the citronella oil that is for garden lamps etc. I got my citronella off EBay as I couldn't find it in any shops locally.